The present invention is generally related to internal combustion engines, and, more particularly, the present invention is related to two-stroke direct fuel-injected engines having thermally-protected piston ring grooves by anodizing. Still more particularly, the invention relates to marine propulsion devices including such engines.
Management of piston/cylinder heat load has become more problematic as improvements in performance of marine internal combustion engines are achieved. For example, the heat load may be large during operation at high engine speeds. More particularly, due to the high temperature generated between one or more piston rings and the respective grooves for receiving such rings, detrimental conditions, such as micro-welding, coaking, and/or excessive wear due to hot scuffing, may develop between the grooves and the piston rings. It will be appreciated that the foregoing conditions may cause degradation in piston ring sealing performance that in turn can lead to undesirable and irreversible coaking of the grooves, higher exhaust emissions and could even lead to shortened useful life of the engine due to engine powerhead failures.
Although U.S. Pat. No. 4,955,353 purports to disclose an anodized layer formed on the lower part of the uppermost piston ring groove to somewhat protect the groove from excessive thermal conditions, that patent shows that the anodizing layer is not contained to within the interior of the groove. It is believed that not containing the anodized layer to only within the interior of the groove results in a diminished heat transfer path between the piston and the cylinder walls where the piston reciprocates. Further, in the specific context of a four-stroke engine, U.S. Pat. No. 5,884,600 appears to disclose a complicated multi-layering arrangement including a hard anodized first layer on the piston""s skirt and piston ring grooves and a composite polymer atop the hard anodized layer on the piston""s skirt. Thus, none of the above-referred patents allows for containing the anodized layer to only within the interior of the grooves and consequently it is believed that the piston of each of the above-referred patents fails to provide an undisturbed heat transfer path between the piston and the cylinder walls at least in the regions where the anodized layer extends outside the interior of the grooves.
Accordingly, it is desirable to provide a piston for an internal combustion engine which eliminates micro-welding and/or excessive wear between the piston rings and the respective grooves that receive such rings, while leaving undisturbed the heat transfer path between the piston and the cylinder wall by containing the hard anodized layer to only within the interior of the grooves. It is further desirable to render the piston applicable to high-output marine engines subject to relatively high combustion temperatures, such as two-stroke direct fuel-injected engines.
Generally speaking, the present invention fulfills the foregoing needs by providing in one exemplary embodiment thereof a marine direct fuel injection internal combustion engine made up of an engine block defining a cylinder having a longitudinal axis and an upper end. A piston is reciprocally moveable in the cylinder along its axis. The piston includes an upper surface portion and a generally cylindrical body extending from the upper surface. A cylinder head includes a lower surface portion closing the upper end of the cylinder to define a combustion chamber between the piston upper surface and the cylinder head lower surface portion. One or more grooves are formed in the piston cylindrical body. Each groove defines an opening to receive a respective piston ring. A thermal-protection coating, such as an anodic coating, is applied to the interior of the uppermost groove. The region of the piston cylindrical body surrounding the groove opening is free of that coating.
The present invention may further fulfill the foregoing needs by providing in another exemplary embodiment a piston for a two-stroke direct fuel injection internal combustion engine coupled to drive a marine vessel. The piston has a generally cylindrical body with two grooves formed therein. Each groove defines an opening to receive a respective piston ring. A thermal-protection coating is applied by anodizing to the interior of the grooves. The respective region of the piston cylindrical body surrounding the respective groove openings is free of that coating.
In another aspect of the invention, a method for improving durability of a two-stroke marine direct fuel injection internal combustion engine is provided. The method allows for anodizing respective surfaces that define one or more respective grooves in a piston of the engine. The groove surfaces define an opening for receiving a respective piston ring, and wherein any exterior region of the piston surrounding the groove opening is free from the coating.